Absolute time synchronization (where the term “absolute” refers to the time-of-day) of nodes in a distributed network is important for many operations, e.g., scheduling of distributed tasks and accurately logging the occurrence of various events. In a radio access network (RAN) application, absolute time synchronization and/or very accurate timing measurements are necessary or desirable in various cellular radio applications like soft and softer handover, diversity operations in general, GPS-assisted positioning, round-trip-time (RTT)-based positioning, etc. Several such applications are now described.
FIG. 1 illustrates a diversity communication in a distributed network that uses a main-remote concept where a single radio base station is split into a main unit and one or remote units. A main base station (BS) unit 10 contains or is associated with a central system clock (CSC). The main base station unit 10 is coupled to a remote base station unit 1 and a remote base station unit 2, (both remote units are labeled 14), via a unidirectional optical fiber ring or loop 12. With diversity communication, a mobile terminal (MT) 16 transmits signals to and receives signals from more than one remote base station unit 14 at the same time. For simplicity, both the main unit and the remote units are referred to as nodes with the understanding that neither a main base station node nor a remote base station node is a stand-alone, complete base station.
Synchronization requirements between the main and remote base station nodes depend on the architectural level of diversity. Consider an example of transmitter diversity in a third generation (3G) mobile radio communications system which employs code division multiple access (CDMA) technology. In CDMA communications, CDMA modulation involves “spreading” information symbols using multiple “chips.” That is why CDMA is also referred to as “spread spectrum” technology. According to the specification for 3G mobile radio communications systems, (3GPP TS 25.104 V5.7.0, section 6.8.4), the absolute time alignment or synchronization error should not exceed ¼ Tc, where Tc represents a “chip” time period, which corresponds 65 nanoseconds (ns).
Another application relates to the Global Positioning System (GPS), which is a satellite-based positioning system providing excellent radio navigation service. GPS can be combined with cellular applications, in which case, it is referred to as Assisted-GPS (A-GPS). For A-GPS, approximately a 5 microsecond (μs) absolute time accuracy is desirable. See, for example, 3GPP TS 25.133 V5.7.0 (section 9.2.10).
Yet another application relates to round-trip-time (RTT) measurements used to determine a location of a mobile terminal. The RTT is the propagation time of a signal traveling from the mobile terminal to the remote base station and back. FIG. 2 illustrates an RTT example where the mobile terminal 16 transmits an RTT signal received by a remote base station node 14, which sends that signal around the fiber loop 12 to the main base station 10 that contains the central clock CSC. The main base station 10 sends that RTT signal around the rest of the fiber loop 12 to the remote base station 14, which then sends it back to the mobile terminal 16. The round-trip time RTT is the time that a signal transmitted from the mobile terminal 16 takes to traverse the radio interface, be received by the remote base station node 14, to be transmitted by the remote base station node 14 back to the mobile terminal 16 over the radio interface, and detected by the mobile terminal 16.
Because the remote base station node 14 is a “dumb” base station node and the “intelligence” of the base station is at the main base station 10, the RTT signal received from the mobile terminal 16 must be routed around the fiber loop 12 through the main base station 10 before it is returned to the remote base station node 14 for transmission back to the mobile terminal. The round-trip delay (RTD) corresponds to the time it takes for the signal transmitted from the remote base station node 14 around the fiber loop 12 to the main base station 10 to be received back at the remote base station node 14. Hence, the RID must be determined and subtracted from the total time in order for the mobile terminal to calculate the actual round-trip time (RTT) measurement. It is desirable that the accuracy of this RTD measurement around the fiber loop be better than ±Tc/2 (±130 ns), according to 3GPP TS 25.133 V5.7.0 (section 9.2.8.1).
In time-of-arrival positioning (TOA), the mobile terminal location/position calculation is based on the propagation delay of the radio signal from the transmitter (the remote base station node 14) to the mobile terminal (MT). When there are at least three TOA measurements available from different remote base station nodes, shown in FIG. 3 as t1-t3, together with other information, e.g., the geographic position of the remote base station nodes, the mobile terminal can determine the geographic position using triangulation calculations. The absolute time synchronization of the three remote base station nodes must be at a level of accuracy on the order of a few nanoseconds. Indeed, a 10 nanosecond uncertainty contributes to roughly a 3 meter error in the RTT-based position estimate, as explained in 3GPP TS 25.305, version 5.4.0, section 9.2
In current radio access networks (RAN) that employ a main-remote base station configuration like that shown in FIGS. 1-3, dedicated communication links are used to connect the main base station node to each remote base station node. Frequency synchronization is obtained using a clock-recovery method based on a phase-locked loop or the like. Absolute time synchronization may be achieved in many node connection topologies using round trip delay (RTD) measurements based on the fact that the uplink from remote-to-main node and the downlink remote-to-main node are symmetrical. When these nodes are coupled to a network with other traffic, switches, routers etc., GPS synchronization may be used to obtain synchronization in the remote base station nodes, assuming each remote radio base station has a GPS receiver either connected to it or in close vicinity.
But there are several drawbacks with topologies that rely on GPS-based synchronization. First, GPS receivers are expensive. Second, GPS synchronization may be less suitable for indoor systems since the GPS signal often cannot penetrate thick walls and cannot be used in tunnels, subways, and the like. Third, some countries may not accept a synchronization solution based on GPS which allows the possibility of the mobile network being effectively disabled if some hostile entity gains control of the GPS system.
As an alternative, an unidirectional fiber optic ring is an attractive network topology for a main-remote base station configuration used in a RAN. It supports synchronous, time division multiplexed (TDM) traffic without additional switches, splitters, add-drop multiplexers, etc. Synchronous traffic advantageously provides inherent frequency synchronization. In addition, a unidirectional fiber ring requires only a minimal number of transceivers in each node—one receiver and one transmitter.
But absolute time synchronization is problematic with unidirectional fiber rings. Although frequency synchronization can be achieved using a standard clock-recovery method, it is not possible to absolute time synchronize individual nodes using roundtrip measurements. This is because the uplink and the downlink are generally not symmetrical in this case, and optical fiber transmission delay is temperature dependent. If the temperature varies even several degrees, the time delay associated with the particular optical length may vary significantly—particularly with respect to the exacting synchronization and/or other time constraints imposed by many applications.
The inventor developed technology that overcomes these temperature-induced timing problems associated with a unidirectional optical fiber ring topology that couples multiple nodes. A round-trip delay time is measured for a signal sent from a first node to travel around the unidirectional optical fiber loop and be received back at the first node. The measured round-trip delay time is used to account for temperature-induced effects on transmissions over the unidirectional optical fiber loop.
In accordance with one aspect of the technology, first and second round-trip delay times are measured with the second round-trip delay time being measured some time after the first. A temperature-induced delay time correction is determined using the first and second round-trip delay times. Based on the determined temperature-induced delay time correction, a time difference is determined between the first node and more and more other nodes coupled to the unidirectional fiber loop. The multiple nodes may then be accurately time-synchronized taking into account the determined temperature-induced delay time correction. As a result of the temperature-induced delay time correction, a timing difference between the synchronized notes is in the range of one nanosecond to several microseconds.
Adjacent nodes in the unidirectional loop are coupled together by an optical fiber link. A time delay is determined for each one of the links. The link time delays are then used to determine the time difference between the first node and the one or more other nodes coupled by the fiber loop. In one example implementation, optical time domain reflectometry (OTDR) is used to determine the time delay associated with each fiber link. But other techniques may be used as well. The temperature-induced time delay correction is based on a difference between the first and second round-trip delay times and the link time delays. The first node may then generate and send a time synchronization message based on a temperature-induced delay time correction for each node to that corresponding node so that the absolute time at that other node is synchronized with the absolute time at the first node.
In another aspect of the technology, one or more remote nodes send a timestamp message to the main node indicating a local time at that remote node. The main node then determines a respective local time difference between the time in each received timestamp message and the local time at the main node. The main node then may use those local time differences in subsequent determinations related to or affected by such local timing differences.
In another example main-remote radio base station embodiment, the main node is a main base station unit that includes processing circuitry and a central clock source. The remote nodes are remote base station units that include radio transceiving circuitry for communicating over a radio interface with a mobile radio terminal. The mobile radio terminal determines one or more round-trip times (RTTs). The RTT corresponds to the time for an RTT message transmitted by the mobile terminal to travel to the remote base station unit and be returned from the remote base station unit to the mobile terminal. The mobile terminal corrects the RTT using an accurately measured round-trip delay time RTD associated with the fiber loop which accounts for current temperature effects on the RTD.